Chlorine dioxide has been commonly used for bleaching pulp since the late 1950s. The process is referred to as the "D" stage of a pulp bleach plant and involves reacting pulp in water with bubbles of chlorine dioxide gas. This causes highly selective destruction of lignin without significant accompanying degradation of cellulose or hemicellulose. The strength of the pulp is preserved, while the pulp is stably brightened. Such bleaching of pulp is described in the following references: The Bleaching of Pulp (Rudra P. Singh, Ed. Tappi Press, 3.sup.rd ed. 1991); Gary A. Smook, Handbook for Pulp & Paper Technologists (Angus Wilde Publications, 2d ed., 1992); Christopher J. Biermann, Handbook of Pulping and Papermaking (Academic Press, 2d ed., 1996); Sven A Rydholm, Pulping Processes (Robert E. Krieger Publishing Co. 1985).
However, part of the chlorine dioxide that is used in the bleaching process becomes entrained in vent gas that is recovered from various locations in the bleach plant. Since chlorine dioxide is an environmental contaminant whose release is subject to regulatory limitations, it must be removed from the vent gas. This is normally accomplished using a scrubber that employs compounds or solutions that destroy the gaseous chlorine dioxide. Suitable chemicals for destroying chlorine dioxide gas include each of sodium thiosulfate, sulfur dioxide, caustic solution, and sulfide. For example, as described in U.S. Pat. No. 5,326,546, chlorine dioxide gas can be removed from a gas stream using a scrubbing solution containing sodium thiosulfate and an inorganic base, such as sodium hydroxide.
Such chemical treatment, however, is expensive. Furthermore, chlorine dioxide that is destroyed in vent gas must be replaced in the D stage of the bleaching operation.
There is therefore a need for a method that avoids loss of chlorine dioxide in vent gas generated in a bleaching operation, and minimizes the need for expensive scrubbing chemicals.